Electronic components such as Read Only Memories (ROMs) utilize devices having extremely high impedance that consume minute amounts of current. These kinds of components are advantageous insofar as the amount of circuitry that can be placed in a small area. Such components have become quite attractive for the above-mentioned reasons and their costs have come down dramatically because of their wide use in the marketplace. These components, however, are quite sensitive to electrostatic discharge (ESD) and special care must be taken in handling. ESD failures were rarely identified a few years ago, except for some very ESD-sensitive technologies such as metal-oxide semiconductor (MOS) and complementary MOS (CMOS). The cost of equipment failure includes not only circuit pack repair costs and field service calls, but, perhaps more importantly, loss of prestige for the product and possibly even the cost of an adverse product liability judgment.
It is possible for a person to inadvertently build up a static voltage in excess of 20,000 volts DC in the course of normal activities. This voltage usually dissipates without notice or harm to an individual; although occasionally, on particularly dry days when voltages can build higher than normal, sudden discharges offer momentary discomfort. While these static voltages present no real harm to humans, they are potentially lethal to certain electronic components unless precautions are taken to limit the flow of current that occurs during discharge.
Modern video games and certain telecommunications equipment provide a versatile menu of functionality by interchanging large quantities of stored data now available in very small volumes such as a ROM. A single piece of hardware is able to operate in a plurality of ways by the mere replacement of the ROM. Uninsulated ROM devices, however, are easily destroyed by ESD if all or part of the ROM is touching ground when ESD is applied.
Several methods for on-chip ESD protection reduce static damage caused by careless handling. ESD-protective networks usually consist of diode or active-device voltage clamps working in conjunction with resistors fabricated on the ROM chip. However, this protection does not come cheaply because valuable chip area, related to device production costs and yield, must be increased. Another problem is that the protection network can reduce circuit performance. In addition, an ESD hit on a device can damage the protection network itself, making it not only subject to latent failure mechanisms, but leaving it in a weakened condition. In this case, the network may fail catastrophically after exposure to a low-level ESD that the device normally would have tolerated.
Assuming that the ESD sensitive component is successfully mounted onto a circuit board, it is still necesary to provide protection from high voltages. One way is to encircle the component with a heavy ground shield. Unfortunately, this solution requires additional space for the ground shield which may also act as an antenna for both transmitting and receiving high frequency signals.
Plastic is an effective insulator, but when a plastic housing surrounds a ROM there are usually seams having small air gaps that provide a discharge path to the ROM. Gluing the seam is effective when it is without gaps, although a cost penalty is associated with the process. The effect of air gaps at a seam can be eliminated by making the plastic housing large enough to provide an approximate half-inch air space between a potential ESD source and the ROM or an electrical path connected thereto. This, however, requires a much larger overall cartridge which is undesirable in today's compact equipment where space is a premium.
It is, therefore, an object of this invention to provide a housing for a ROM or similar device that provides protection from ESD.
It is another object of this invention that the housing be of minimum dimensions.
It is yet another object of the invention to provide a housing for a ROM device that may be assembled in a cost effective manner.